Intramedullary nails for long bone fracture setting

ABSTRACT

An intramedullary nail has a proximal part for engagement with a proximal bone fragment and a distal part for engagement with a distal bone fragment. A motion assembly interconnects the parts and allows limited axial relative motion of the proximal and distal parts. This limited axial motion provides micromotion and in some embodiments also dynamization. The nail may comprise a stem for fastening to one bone fragment and an insert within the stem for fastening to the other bone fragment, the insert being adapted to guide insertion of a bone screw through the stem and to prevent relative rotation of the distal and proximal bone fragments. The motion assembly may include spring bias and/or damping means between the parts, possibly including one or more Belleville washers.

FIELD OF THE INVENTION

The invention relates to setting of long bones after fractures.

PRIOR ART DISCUSSION

Fractures of the long bones can be set using an Ilizarov frame (which isexternal fixation) or using an intramedullary nail (inserted into thehollow canal of the long bones). As shown in FIG. 1, the Ilizarov frameallows the bones to move in a controlled fashion along the longitudinalaxis. This motion stimulates healing. The bone fragments are held inplace by wires penetrating the skin and attaching to an external frame.The tension in the wire determines the amount of motion between the bonefragments. When loads are placed on the fragments they compress, andwhen the load is removed they return to their original set position. Ingeneral, the Ilizarov frame also admits some rotational motion and theaxial and rotational stiffness are highly dependent upon theconfiguration of the rods and wires.

Intramedullary nails (“IM” nails) are hollow tubes placed internallyinside the fractured bone. Screws passing through the bone holdfragments in place at a set distance apart. There may be delayed unionor non-union of the bone fragments. When this occurs, a second operationis undertaken whereby a screw is removed to allow one of the bonefragments to move relative to the others and close the gap, promotingcompression and hence bone healing. This is called “dynamisation”.Dynamisation requires a surgical procedure, with the disadvantages ofrisk to the patient undergoing anaesthesia and cost of the procedure.Also, as the gap will be closed this may in extreme cases lead to limbshortening, which can be a side-effect of the dynamisation procedure.Limb shortening occurs due to the dynamic bone fragment not being ableto return to its set position.

Examples of published documents in this field are Chinese PatentPublication Nos. CN1875892 and CN2820114.

The invention is directed towards achieving improved long bone setting.

SUMMARY

According to the invention, there is provided an intramedullary nailcomprising a first part having a first opening to receive a firstfixture for engagement with a first bone fragment. There is a second,part having a second opening to receive a second fixture for engagementwith a second bone fragment. A motion assembly allows limited axialrelative movement of the first and second parts.

In one embodiment, the second part is a nail stem and the first part isan insert within the stem.

In one embodiment, the insert is adapted to guide insertion of a bonescrew through the stem.

In one embodiment, the insert is constrained to move axially only withinthe stem without relative rotation of the insert and the stem. In oneembodiment, the insert is keyed in the stem.

In one embodiment, the motion assembly is adapted to provide spring biasand/or damping between the parts.

In one embodiment, the motion assembly comprises a removable spring. Thespring may comprise a plurality of spring elements. The motion assemblymay be adapted to accommodate any of a range of spring elements topre-set a bias range. The spring or spring elements may comprise one ormore Belleville washers.

In one embodiment, the motion assembly is adapted to allow movementwithout bias or damping.

In one embodiment:

-   -   the second part comprises a nail stem with an aperture,    -   the first part comprises an insert within the stem, the insert        being constrained to move axially only within the stem;    -   the motion assembly comprises the insert, the stem, and one or        more bone screws for engagement with a first bone fragment, and    -   wherein there is a difference in cross-sectional size between        the bone screw and the aperture, said difference allowing        movement.

In one embodiment, the motion assembly comprises a dynamisationadjustment mechanism for allowing adjustment of the interfragmentary gapwithin which said movement occurs.

In one embodiment, the dynamisation adjustment mechanism is adapted toadjust axial location or range of permitted axial locations within anail stem of an insert for attachment to a bone fragment, the nail stembeing for attachment to the other bone fragment.

In one embodiment, the dynamisation adjustment mechanism comprises aretainer in the stem, axial position of the retainer being adjustable toset a limit on motion of the insert relative to the stem.

In one embodiment, the retainer engages threads in the stem, rotation ofthe retainer setting its axial position.

In one embodiment, the dynamisation adjustment mechanism is adapted tovarying minimum and/or maximum separation of the proximal and distalbone fragments.

In one embodiment, the adjustment mechanism is also adapted to adjustspring bias.

In one embodiment, the motion assembly allows movement under springbias, and upon full spring compression the dynamisation adjustmentmechanism is adapted to allow de-compression of the spring whilebringing the proximal and distal parts closer together.

In one embodiment, the dynamisation adjustment mechanism is adapted tolock a position between a bearing and the stem, the bearing being actedupon by the insert via the spring.

In one embodiment, the bearing is a sleeve within which the insert islocated.

In one embodiment, the dynamisation adjustment mechanism is adapted tooperate upon patient application of weight.

In another embodiment, the dynamisation adjustment mechanism is adaptedto allow progressive relative axial mutual movement of the parts forprogressive reduction of an interfragmentary gap.

In one embodiment, the dynamisation adjustment mechanism comprises aratchet mechanism and adjustment is from one ratchet position toanother.

In one embodiment, the dynamisation adjustment mechanism comprises aseries of grooves in a nail stem and a ratchet for engagement in thegrooves.

In a further embodiment, the ratchet comprises a radial springconfigured to engage a ratchet groove.

In one embodiment, the dynamisation adjustment mechanism is adapted toallow relative motion by engagement of a pin in a slot, in which the pinis not breakable under normal conditions.

In one embodiment, the dynamisation adjustment mechanism includes a linkhaving a failure level of applied force.

In one embodiment, the link is arranged to be under shear force and tofail at a threshold shear force.

In one embodiment, there is a first range of relative motion beforefailure and a second range after failure.

In one embodiment, a link extends through a short slot and a second linkextends through a longer slot.

In one embodiment, the nail comprises a surgeon adjustment interfacewhich translates rotational movement caused by a surgeon into axialmovement within the stem to close the gap between the proximal anddistal bone fragments.

In one embodiment, the interface comprises an exposed screw which pushesan internal sliding and non-rotating component within the stem.

DETAILED DESCRIPTION OF THE INVENTION Figures

The invention will be more clearly understood from the followingdescription of various embodiments thereof, given by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a perspective view of a prior art external frame arrangementas set out above;

FIG. 2 shows a nail of the invention in use and FIG. 3 shows an explodedview of the nail;

FIG. 4 is a cross-sectional view showing an upper part of the nail, andFIG. 5 is a cross-sectional view showing a stem of the nail in moredetail

FIG. 6 shows an insert for engagement with a proximal bone fragment;

FIGS. 7 and 8 are perspective and side views of a Belleville springwasher for micromotion bias;

FIG. 9 is a perspective view of a pusher piece to set an initialposition within the nail;

FIG. 10 is a perspective view of a radial spring providing discretepositions within the nail when it engages with one of the grooves in thestem;

FIG. 11 is a diagrammatic cross-sectional view showing a ratchet whichcarries the radial spring, the proximal end of a sleeve within the stem,and how they engage with each other to provide micromotion autoadjustment;

FIG. 12 is a view similar to that of FIG. 11, showing an alternativearrangement;

FIGS. 13 and 14 are diagrams showing operation of the nail of FIGS. 2 to11, particularly how micromotion automatic adjustment is achieved,

FIG. 15 shows an alternative nail of the invention from the top, front,and right views, as well as a section view;

FIG. 16 is a cross-sectional view of part of the nail; and

FIG. 17 is a set of exploded assembly drawings of this nail including aleft view, an isometric view, and a sectioned isometric view;

FIG. 18 is a set of views showing a variation on the embodiment of FIG.15 in which the end cap is extended to form a plug filling the centrallumen of the nail;

FIGS. 19 and 20 show a variation in which Belleville washers are addedto control swing-phase micromotion;

FIGS. 21 and 22 show a variation in which Belleville washers are addedto control stance-phase micromotion;

FIGS. 23 and 24 show a variation on the embodiment of FIG. 15 in whichthe device is comprised of an anchor part, positioned inside a nailcentral lumen before the nail is implanted into the patient viaattachment to an insertion handle used in a standard operativetechnique, and a modified end cap that mates with the anchor and isinserted after the nail is implanted;

FIGS. 25 to 39 are views in the same general formats of still furtherembodiments;

FIGS. 40 to 57 are views shows six further embodiments comprisingalternative configurations of the mechanical subcomponents described inthe embodiments of FIGS. 15 to 19; and

FIG. 58 is a diagram illustrating alignment of a bone screw through anail stem in such a way as to admit controlled micromotion.

DETAILED DESCRIPTION

Recent advances in medicine have demonstrated that accelerated healingof long bone fractures can be induced by mechanical stimulation of theinterfragmentary gap in a process described clinically as “micromotion”.Described herein is an intermedullary nail that includes a first part, asecond part and a motion assembly to allow limited axial relativemicromotion of the parts. According to one type of approach, the firstpart (such as an insert) includes a first opening to receive a firstfixture (such as a bone screw) that is secured to a first bone fragment.The second part (such as a nail body) has a second opening to receive asecond fixture (such as another bone screw) that is secured to a secondbone fragment. The second part has an inner diameter that is larger thanthe first part's outer diameter so that the first part fits inside thesecond part. The second part has a third opening that also receives thefirst fixture. The third opening has an axial length to establish amaximum range of distances between the first part and said second part.

In some embodiments, the “motion assembly” is a set of parts which forma chain of interconnection between the first and second parts, includingfor example springs such as Belleville washers. In other embodiments the“motion assembly” constitutes the first and second parts, configured toallow controlled relative movement between the parts.

In clinical practice, a small percentage of fractures fail to unite dueto a variety of factors including the type of fracture, degree of softtissue injury, torsional stability of the fixator, distance between bonefragments after fixation, and patient risk factors such as smoking.Delayed union or non-union of the bone fragments is commonlycharacterised by a large interfragmentary gap (typically more than about3 mm). In such a situation micromotion occurs insofar as the proximaland distal bone fragments move relative to each other, but because theyare so far apart healing does not occur.

This problem can be dealt with by modifying the fixator so as to reducethe gap between the bone fragments to the range associated with positivehealing outcomes. Fixator modification is typically irreversible andallows dynamic closure of the fracture gap, so this is referred toclinically as “dynamisation”. In this embodiment, the motion assemblycomprises components which can produce stimulatory micromotion anddynamisation in case of fracture non-union.

In this specification the terms “proximal” and “first” are usedinterchangeably, as are the terms “distal” and “second”.

Referring to FIGS. 2 to 11 an exemplary intramedullary nail (henceforth“nail”) 1 has a stem 7 and is inserted in a long bone B across afracture, joining a first bone fragment part (such as a “proximal” partas hereafter described) and a second bone fragment part (such as a“distal” part as hereafter described). According to the exemplaryapproach depicted in FIGS. 2 to 11, the proximal end of the nail hasslots 2 and 3 in the stem 7. Screws are inserted through the slots 2 and3 and engage a motion assembly (depicted as an insert 30 in FIGS. 2 to11) within the stem 7 as described in more detail below. The lower shankof the stem 7 has through-holes 8 and 9 for fixed engagement of the stem7 with the distal bone fragment.

The distal end of the stem 7 is fixed to the distal bone fragment byscrews through the holes 8 and 9. The insert 30, within the stem 7, issecured to the proximal bone fragment by engagement with screws 4 and 5through apertures 31 and 32.

The insert 30 with respect to the stem 7 is movable to a limited extentunder spring bias and damping to provide micromotion. “Micromotion” isregarded in this specification as a small interfragmentary motion (i.e.less than about 1.5 mm) that is allowed to occur repeatedly. Controlledmechanical stimulation of the interfragmentary gap via application ofmicromotion in the direction of the long bone axis has been shown inanimal experiments and human-subjects clinical trials to accelerate theproliferation of callus at the fracture site and reduce time to clinicalunion of the bone fragments.

Returning to FIGS. 2 through 11, the embodiment of the nail 1 depictedtherein includes:

-   -   The stem 7 having, at the proximal end, internal ridges 15,        internal threads 16, and slots 2 and 3.    -   A sleeve 20 having an elongate slot 21 and being configured for        insertion into the stem 7, and to house the insert 30 and        Belleville washers 33, providing a bearing surface for the        washers 33.    -   The insert 30 having the through-holes 31 and 32.    -   Belleville washers 33 around a narrower length 34 of the insert        30, and being engaged between the insert 30 and the sleeve 20 at        its lower end.    -   A ratchet 40 carrying a radial spring 60 engaging the ridges 15        of the stem 7.    -   A pusher piece 50, engaging the threads 16 of the stem 7 and        pushing against the ratchet 40.

The insert 30 embodiment depicted in FIG. 3 consists of the following:

-   -   A hollow space region 35 (hereinafter referred to as “Central        lumen 35”) to accommodate a guide-wire during the clinical        procedure. The nail preserves the central lumen for guide wire        usage during nail insertion. Some embodiments may be a two-part        assembly wherein a second part is added later that blocks the        guide wire, but this occurs after the guide wire has been        removed.    -   A keying mechanism to ensure rotational alignment with the        external insert assembly. Screw holes 31 and 32 to accommodate        fixation screws 4 and 5. The screw hole locations are aligned        with the slots 2 and 3 in the stem 7.    -   The distal section of the insert 30 has a reduced diameter 34        and shoulder to facilitate the inclusion of a spring        mechanism/shock absorption system 33 implemented with (in the        embodiment of FIG. 3) Belleville washers 33. This reduced        diameter section mates with a hole at the base of the sleeve 20        such that vertical movement of the insert 30 is accommodated.        This arrangement will facilitate micromotion of the insert 30 as        it sits on the Belleville washers 33, exerting force via the        washers onto the base of the sleeve 20.

The insert 30 and the washers 33 are placed in position before theratchet assembly 60 and 40 is screwed in place at the top of the outersleeve 20. The insert 30 central lumen 35 allows passage of a guidewire. The two holes 31 and 32 allow fastening of the insert 30 by bonefastener screws 4 and 5 to the proximal bone fragment. The screws 4 and5 pass through the keyed groove 21 in the sleeve 20. The narrowerdiameter length 34 of the insert 30 allows the insert 30 to sit on theBelleville washers 33 and provides a centring mechanism for the washers33.

The washers 33 provide vibration damping and restoring force for theinsert 30 as weight/downward force is applied to the insert 30 by theproximal bone fragment. The Belleville washers 33 are particularlyadvantageous in the nail 1 due to their high spring constants,suitability to confined space, large load capability, and shortdisplacement. Various combinations of these washers can be used to givea wide range of displacement and restoring forces. Although Bellevillewashers are specifically depicted in the particular embodiment of FIG.3, in alternate embodiments other elements may be employed to effect thespring mechanism/shock absorption component. Examples include coned-discwashers, wave washers, slotted washers, finger washers, split washers,curved washers, volute springs, or coil springs (standard, variablepitch, barrel, hourglass, or conical). For simplicity, the remainder ofthe description will largely refer to Belleville washers rather than aspring mechanism/shock absorption component.

As shown particularly in FIG. 11, the ratchet 40 has a tapered profilewhich mates with a negative tapered profile of the proximal end of thesleeve 20. A ratchet surface 70 mates with a sleeve surface 80, and aratchet surface 72 mates with a sleeve surface 82, preventingde-coupling of the ratchet 40 from the sleeve 20 in use.

In another embodiment, as shown in FIG. 12, a sleeve 20(a) has ashoulder 83. This arrangement of profiles allows insertion of Bellevillewashers 33 at this location in addition to or instead of the location atthe distal end of the insert 30. In a further alternative embodiment,the function of the sleeve 20 or 20(a) could be performed by a solidpiece, having slots to accommodate the bone-fixing screws.

The insert 30 has a limited freedom of movement under bias of thewashers 33 with respect to the stem 7. The force exerted by the washers33 are set by the following:

-   -   (a) Initial choice by the surgeon of the appropriate assembly is        based on patient weight. The surgeon is provided with a choice        of micromotion assemblies to use for any particular operation.        The full assembly including Belleville washers 33, insert 30,        sleeve 20, ratchet mechanism 40/60 is factory set and labelled        according to patient weight. The overall spring property of the        set of Belleville washers 33 is determined by geometrical        properties such as washer height, inner/outer diameter, and        material thickness. Multiple washers can be arranged in parallel        or series configurations to provide the specific spring        properties required for a given patient weight class.        Interaction effects such as friction between Belleville washers        are well-known and can be accounted for in the labelling of the        assembly according to patient weight class. The surgeon does not        need to perform any assembly work.    -   (b) Adjustment of the axial position of the pusher piece 50        within the stem 7. The adjustment is achieved by rotation of the        pusher piece 50 on the threads 16. This movement pushes the        ratchet 40/60 into engagement with a next groove 15, and the        ratchet 40 engages the proximal end of the sleeve 20.

The chain for interconnection of the proximal bone fragment to thedistal bone fragment is as follows:

-   -   screws 4, 5;    -   insert 30,    -   Belleville washers 33,    -   sleeve 20 (urged downwardly by the washers 33);    -   ratchet 40;    -   ratchet spring 60 snap-fitted into one of the stem 7 grooves 15,        the stem 7, secured to the distal bone fragment at the screw        holes 8 and 9.

In the surgical procedure the surgeon chooses a particular nail to suitthe weight and size of the patient. The surgeon will then choose theappropriate motion assembly which will be labelled according to patientweight and put the motion assembly into the nail 1 during the surgicalprocedure. This nail 1 will have the appropriate number of Bellevillewashers 33 (as described above). In the surgical procedure the surgeonrotates the pusher piece 50 to urge the ratchet 40/60 down to snap-fitinto an appropriate groove 15. This in turn pushes down the sleeve 20.

FIGS. 13 and 14 show diagrammatically operation of the nail 1 to achieveautomatic micromotion adjustment. FIG. 13 shows the nail 1 after thesurgical procedure, with the Belleville washers 33 fully compressed. Onapplication of weight the insert 30 cannot move further and so theinsert 30 and sleeve 20 move as one unit pulling the ratchet 40/60 tothe next groove 15, provided enough force is applied to overcome theratchet forces. FIG. 14 shows the position after this automaticadjustment, and the fracture between the proximal and distal bonefragments has closed. The sleeve and the insert 30 have moved downwards,as has the proximal bone fragment relative to the distal bone fragment.The Belleville washers 33 are no longer compressed because the sleevehas moved down relative to the stem 7. Thus, the invention aids thebone-healing process, for example for tibia fractures or femoralfractures.

During leg swing phase, the tendency is for the bone fragments toseparate therefore releasing the compressive forces on the Bellevillewashers 33 such that they tend to return to their original profile. Theextent to which the bone fragments can separate is maximised by theexistence of the ratchet mechanism 40, which serves as a hard stop forthe motion assembly (and therefore bone fragment separation process)during the leg swing motion.

It will be appreciated that the invention of at least some embodimentsachieves micromotion between bone fragments, while maintaining theoverall stability achieved by an intra-medullary device, accuratesetting of bone separation distances during the surgical procedure,non-invasive correction of bone fragment separation distancespost-operation in the event that the bone fragment separation distanceswere miscalculated during the procedure, and/or prevents post-operativebone fragment separation during leg swing phase.

The ratchet mechanism 40 allows one to increment the motion assemblydown into the intra-medullary nail in a controlled fashion. This willallow the surgeon to accurately locate the insert 30 within the nail 1as well as move the insert 30 incrementally down into the stem 7 onapplication of a force. The ratchet facilitates accurate location of theinsert 30 and hence accurate separate distance of apposing bonefragments during the surgical procedure. Also, the ratchet mechanism 40aids progressive compression at the fracture site. The internal screwthreads 16 accommodate the mating screw thread 51 on the pusher piece50. Ability to screw in the pusher piece 50 means that the verticallocation of the insert 30 can be controlled by controlling the amount bywhich the pusher piece 50 is screwed down into the sleeve 10. Slots maybe in either the proximal or distal locations or both.

The following describes other embodiments.

The pusher piece may or may not be attached to the motion assembly 30.It is used to push the sleeve in a controlled and measured fashion. Thebottom of the pusher piece butts against the top of the sleeve or theratchet either directly of by use of an interposition device such as aspring. The pusher piece has an internal cental lumen to facilitatetracking of a guide-wire during the procedure. The rotational action ofthe pusher piece is translated into vertical motion of the motionassembly 30. Alternative methods for inserting the pusher piece may beemployed, such as direct vertical force and clicking into a slot. Theinsert and the sleeve must not rotate as its rotational orientation mustremain constant so as to align it with the slots in the stem. This isfacilitated by keying the insert assemblies with the inside of the nail.

Rotational stability of the insert and the outer sleeve can bemaintained by keying mechanisms or other appropriate rotationalstability mechanisms. An objective is to ensure alignment of the screwslots and screw holes in the nail 1 and the insert. For this to happen,the rotational motion of the pusher piece is translated into verticalmotion of the insert with no possibility of rotation of the insert 30once it has engaged. If the pusher piece is inserted by direct verticalforce or by clicking into a slot, rotational alignment of theinsert/assembly should be maintained.

The ratchet is such that a threshold force is exceeded for the insert tomove from one ratchet position to the next. This may be a function ofratchet pitch or spring stiffness. The distance between ratchet groovesprovides accurate setting of bone fragment separation during theprocedure. High resolution in the mechanism for accurate setting andmaintenance of bone fragment separation distance and resultant optimumfracture union.

The ratchet prevents the outer sleeve and insert from migrating up thenail. Positioning of the outer sleeve and therefore the insert isachieved initially by means of screwing/pushing/clicking the pusherpiece down into the nail and thereafter (auto adjustment) by having thepatient apply weight until a threshold weight is exceeded.

Keying mechanisms operate to:

-   i. retain correct relative orientation of the sleeve and the insert    during insertion into the nail stem 7, and-   ii. interface with the internal diameter of the tibia nail.

A full length slot (or plurality of slots) facilitates the dynamicfixation process. The slot in the sleeve lines up with the dynamic slotsin the nail stem and the screw holes in the insert. These slots may beparallel, or in sequence or in differing positions on the nail. It isimportant to retain rotational alignment between the slots in the nailand in the insert/sleeve assembly and the screw holes in the insert.Keying mechanisms ensure alignment while the motion assembly is beinginserted into the nail.

Referring to FIGS. 15 to 17, only the proximal end of an intramedullarynail 100 of another embodiment is shown. The nail 100 is shown insimplified form with a single stem slot 101 by way of example. The nail100 has a stem 110 with a short threaded section 102 in the proximalend, which may receive likewise threaded components including but notlimited to a motion assembly 103 incorporating both micromotion anddynamisation capabilities and an end cap 104.

In the embodiment shown in FIGS. 15 to 17, an inner insert 106 and anouter insert 107 are pre-assembled using two pairs of shear pins 108 and109 for dynamisation. The motion assembly 103 (shown most clearly inFIG. 15) is inserted into the nail stem 110 and secured to the internalthreads 111 in the proximal end of the nail stem 110 via externalthreads 112 in the top of the outer insert 107 (shown most clearly inFIG. 17). A proximal bone fragment is secured by engagement with a bonescrew 115 inserted through the slot 101 in the nail stem 110 and a hole116 in the inner insert 106. The end cap 104 is secured inside the nailstem 110 by engagement with the internal threads 111 in the proximal endof the nail stem 110 and/or the outer insert.

After the motion assembly 103 is secured inside the nail stem lumen, theinner insert 106, which is attached to the proximal bone fragment viathe bone screw 115, is movable with respect to the nail stem 110, whichis attached to the distal bone fragment via the distal bone screw. Thisrelative motion comprises three phases:

-   -   Phase I (Micromotion)—The dynamisation unit 103 allows        controlled bone fragment separation as the upper shear pin pair        108 travels in an upper slot 120 in the inner insert 106. The        interfragmentary micromotion distance is chosen to accelerate        callus formation and fracture healing by mechanical stimulation.        The motion assembly 103 operates in Phase I until a defined        threshold load is applied by the patient by bearing weight on        the leg containing the device. Above the threshold load, the        upper shear pins 108 will be severed by the shearing action of        the inner insert 106 and outer insert 107.    -   Phase II (Partial Dynamisation)—In the event that non-union        occurs the patient can be asked to apply weight to the bone, to        thereby cause dynamisation to occur. This is because the upper        shear pins 108 fail, and the motion assembly 103 allows        controlled bone fragment separation as the lower shear pin pair        109 travels in a lower slot 121 in the inner insert 106. The        partial dynamisation distance is chosen to improve fracture        apposition to promote healing in cases of delayed union or        non-union. The motion assembly 103 operates in Phase II until a        defined threshold load is applied by the patient by standing on        the leg containing the device. Above the threshold load, the        lower shear pins 109 will be severed by the shearing action of        the inner insert 106 and outer insert 107.    -   Phase III (Full Dynamisation)—After both pairs of shear pins 108        and 109 have failed, the separation of bone fragments is        controlled by translation of the bone screw 115 in the slot 101        in the proximal end of the nail stem 110 a distance dictated by        the length of the slot 101 in the nail stem 110.

Hence, the device described in this embodiment achieves mechanicalstimulation for accelerated healing via interfragmentary micromotion,controlled bone separation during both stance and swing phases viamechanical hard-stops, and multi-stage non-surgical dynamisation toprovide controlled apposition of the proximal and distal bone fragments.

In more detail, the motion assembly 103 comprises:

-   -   The inner insert 106 having a larger-diameter lower end for        insertion into the lumen of the nail stem 110, a        smaller-diameter upper end for insertion into the outer insert        107, an inner diameter tapered at the proximal end, a        through-hole 116 for the bone screw 115, and two pairs of slots        120 and 121 for the upper and lower shear pin pairs 108 and 109.    -   The outer insert 107 having external threads 112 for attachment        to the internal threads 111 of the nail stem 110, a tooling slot        137 to allow insertion of the dynamisation unit 103 into the        nail stem 110 using a surgical screwdriver or similar        instrument, and through-holes 130 and 131 to hold the upper and        lower shear pin pairs 108 and 109.    -   The upper shear pin pair 108 having a uniform cross-section or        having a groove or other stress concentrating feature at the        sliding interface between the inner insert 106 and outer insert        107, depending on the desired threshold load limit for        dynamisation from Phase I to Phase II as described above.        Varying the diameter of the shear pins and/or the shape and size        of the groove can be used to give a wide range of threshold        loads for pin failure.    -   The lower shear pin pair 109 of the same or similar design as        the upper shear pin pair 108, depending on the desired threshold        load limit for dynamisation from Phase II to Phase III as        described above.    -   The end cap 104 having a threaded end for attachment to the        internal threads 111 of the nail stem 110, a smaller diameter        end to interface with the inner diameter of the outer insert        107, and a tooling slot 138 to allow insertion into the nail        stem 110 using a surgical screwdriver or similar instrument.

The motion assembly 103 preserves a central lumen 135 to allow thepassage of a guide wire during the surgical procedure. The taperedproximal end of the inner insert 106 provides a smooth transition fromthe larger diameter of the nail stem lumen 136 to the smaller innerdiameter of the dynamisation unit 103.

The motion assembly 103 allows controlled bone separation (micromotion)by limited freedom of movement of the inner insert 106 relative to theouter insert 107 as constrained by the upper shear pin pair 108 duringPhase I or the lower shear pin pair 109 during Phase II. The loadrequired to cause failure of the shear pins is determined by the sizeand cross-section of the shear pins and/or the shape and size of anystress-concentrating feature, so the shear pin characteristics can thusbe tailored for a specific patient weight class. The surgeon is providedwith a choice of motion assemblies, which are assembled, sterilised, andlabelled according to patient weight. Before inserting the nail 100 intothe patient, the surgeon chooses the appropriate motion assembly andsecures it in the nail stem 110 by mating of the internal threads 111 inthe nail stem 110 with the external threads in the motion assembly. Thesurgeon checks that the hole 116 in the inner insert 106 and the slot101 in the nail stem 110 are aligned so as to freely accept the bonescrew 115. This check may be accomplished by simply inserting the bonescrew 115 through the nail 100 with the motion assembly 103 installedand adjusting the rotational position of the motion assembly 103 using asurgical screwdriver or similar instrument engaged in the tooling slot137 in the outer insert 107. The surgeon may then proceed with thestandard surgical procedure for inserting the nail 100 into the patient.

After surgery, the chain for interconnection of the proximal bonefragment to the distal bone fragment is as follows:

-   -   proximal bone screw 115;    -   inner insert 106;    -   upper shear pin pair 108 (if in Phase I) or lower shear pin pair        109 (if in Phase II);    -   outer insert 107;    -   nail stem 110, secured to the distal bone fragment at screw        holes (not shown).

During the stance phase, the bone fragments tend to move together as thepatient applies weight on the injured leg. During this phase, the distalbone fragment is considered fixed. The patient's body weight istransmitted through the proximal bone screw 115 to the inner insert 106,which slides downward relative to the shear pins 108 and 109. In PhaseI, the upper slot 120 of the inner insert 106 bears downward on uppershear pin pair 108, which transmits the load to the outer insert 107. InPhase II, the lower slot 121 of the inner insert 106 bears downward onthe lower shear pin pair 109, which transmits the load to the outerinsert 107. The outer insert 107 is attached to the nail stem 110 viascrew threads, so the load travels through the nail stem 110 to thedistal bone screw(s) and distal bone fragment. In Phase III, when allshear pins have failed, the inner insert 106 travels freely relative tothe outer insert 107 until the bone screw 115 bears down on the bottomof the slot 101 in the nail stem 110.

During the swing phase, the bone fragments tend to move apart as thepatient takes weight off the injured leg. During this phase, theproximal bone fragment is considered fixed. The combined weight of thefoot and shank pulls downward on the distal bone screw(s) and this loadis transmitted to the nail stem 110 and the outer insert 107 via thescrew threads 111 and 112. In Phase I, the outer insert 107 and bothshear pin pairs 108 and 109 translate downward until the pins bear downon the slots 120 and 121 in the inner insert 106. In Phase II, after theupper shear pins 108 have failed, only the lower shear pins 109 beardown on the lower slot 121 in the inner insert 106. The foot and shankweight is thus transmitted through the pins, the inner insert 106, andthe bone screw 115 to proximal bone fragment. In Phase III, when allshear pins have failed, the outer insert 107 travels freely relative tothe inner insert 106 until the bone screw 115 bears upward on the top ofthe slot 101 in the nail stem 110.

This embodiment facilitates several improvements including some or allof allowing axial interfragmentary micromotion to stimulate callusformation at the fracture site and accelerate healing (Phase Ifunctionality), controlling the interfragmentary displacement in boththe stance and swing phases, allowing for non-surgical post-operativeadjustment of bone separation distance, or dynamisation (Phases II/IIIfunctionality), and/or achieving the aforementioned improvements whilemaintaining the torsional and bending rigidity of a standard IM nail.

The following describes other aspects of the illustrated embodiments andvarious alternative embodiments.

In one embodiment, the nail may include a plurality of slots or otherfixture holes in the proximal end to accommodate a plurality of bonescrews or other fixtures. The slots or fixture holes may be arranged ina single plane or in a series of oblique planes rotated about thelongitudinal axis of the nail. The inner insert may contain a pluralityof through-holes for a plurality of bone screws or other fixturesaligned with the slots in the proximal end of the nail.

In one embodiment, the dynamisation unit may be attached to the nailstem by screw threads or by one or a plurality of pins or otherfasteners positioned in slots, holes or openings in the proximal end ofthe nail, which has been shown in simplified form in FIGS. 15 to 17, forillustrative purposes only.

In one embodiment, the nail may be cast, machined, or otherwise formedso as to include any or all of the features of sub-components described(including but not limited to the outer insert and shear pins) as anintegrated single part while achieving the same or similarfunctionality.

In one embodiment, the dynamisation unit may be mounted in the distalend of the nail to achieve the same or similar effects as described inany other embodiment herein.

In one embodiment, the shear pins may comprise a non-circularcross-section, including but not limited to any n-sided prismaticpolyhedron, or any elliptic, parabolic, or hyperbolic cylinder. Theshear pins may be symmetric or asymmetric about any axis and maycomprise a combination of cross-sections. The pins may also take theshape of a parallel, gib head, tapered, or woodruff key, or any otherdemountable machinery part that may be assembled into a keyseat or otherreceiving slot to provide a positive means of transmitting force betweentwo components. These keys may also be used to provide rotationalalignment of the insert and stem and additional torsional stability. Thepins may or may not contain stress-concentrating features such asgrooves, notches, depressions, divots, or step-changes in geometry. Thepins may be formed from one piece of material machined or otherwiseshaped to give the desired shape, or they may be formed of a pluralityof pieces and or materials joined together to form the desired shape.

In one embodiment the surface of the inner insert 106 may incorporatetwo flat areas to facilitate a perpendicular shearing interface relativeto the shear pins 108. This will result in a more uniform loadingthrough the dynamisation phases I-III.

In one embodiment, the bone fragment separation distance (micromotion)facilitated in Phase I may be altered to any distance desired bychanging the length of the upper slot 120 in the inner insert 106.

In one embodiment, the bone fragment closure distance (partialdynamisation) facilitated in Phase II may be altered to any distancedesired by changing the length of the lower slot 121 in the inner insert106.

In one embodiment, the invention may comprise more than two shear pinpairs to achieve a plurality of micromotion/dynamisation phases fornon-surgical adjustment of bone separation distance. The plurality ofpin pairs may be of the same or similar design and may be arranged in asingle plane or in a series of oblique planes rotated about thelongitudinal axis of the nail.

In one embodiment, dynamisation may be achieved by the patient bearingweight statically or by impact loading, with or without assistance fromthe surgeon or another person, or by any other means of generating asufficient force component directed along the longitudinal axis of thenail so as to achieve failure of one or more shear pin pairs.

In one embodiment, the shear pins may be encapsulated within or insertedinto a bearing sleeve comprised of a viscoelastic, hyperelastic, rubber,or other energy-absorbing material to cushion the impact of the pins ontheir bearing surfaces in the slots and in the inner insert. In afurther embodiment, the energy-absorbing material may be affixed to thebearing surfaces of the inner insert.

In one embodiment, the shear pins may be press-fit, welded, threaded, orotherwise rigidly affixed to the outer insert to achieve pin fragmentcontrol after dynamisation (Phases II and III).

In one embodiment, a biocompatible radiopaque material, including butnot limited to platinum or tantalum, may be embedded in or attached toone or more components previously described in order to determine theirposition relative to one another, the bone screw(s), or the IM nail.

In one embodiment, the device may also comprise an integrated electronicor magnetic sensor or indicator to detect or measure axial movementwithin the device (e.g. interfragmentary motion, micromotion,dynamisation), which may be indicative of fracture healing or otherclinically-relevant outcome.

In one embodiment, a nail 150 has an end cap 151 extended as illustratedin FIG. 18, to form a plug for the central lumen 152 of the dynamisationunit 103, so that shear pin fragments generated during the transitionfrom Phase I to Phase II/III are prevented from falling into the centrallumen of the dynamisation unit.

In a further embodiment, the lumen plug may comprise a componentinserted into the device separately from the end cap and threaded orotherwise secured to the inner insert and/or the outer insert, and/orthe subsequently-attached end cap. In a further embodiment, the lumenplug may be an integrated feature of any existing component to achievethe same or similar functionality.

In one embodiment a nail 159 (FIGS. 19 and 20) has a Belleville washeror a plurality of Belleville washers 160 located between an inner insert166 and the outer insert 167. The Belleville washers 160 are constrainedbetween the top surface of the inner insert 166 and an added lip 161 onthe top edge of the outer insert 167. The Belleville washers 160compress during the swing phase when the weight of the shank and footcause the nail stem 110, outer insert 167, and shear pins 108 and 109 totranslate downward relative to the proximal bone fragment, bone screw115, and inner insert 166.

In a further embodiment (FIGS. 21 and 22) in a nail 180, a Bellevillewasher or a plurality of Belleville washers 181 may be added between thebone screw 115 and an inner insert 186 to provide damped micromotion.The Belleville washers are seated on a bearing lip 182 of the innerinsert, which is then inserted into an outer insert 187 and secured bythe shear pins 108 and 109 to form the pre-assembled dynamisation unit.The dynamisation unit is inserted into the nail stem 110, which is thenimplanted in the patient as previously described. There may or may notbe a spacer ring, flat washer, or other insert positioned on top of thestack of Belleville washers 181 to maintain their position duringinsertion of the bone screw 115. The bone screw passes through a slot188 in the inner insert 186 to allow micromotion. During the stancephase, the patient's weight is transmitted by the bone screw 115 throughthe Belleville washers 181 to the inner insert 186. As the Bellevillewashers 181 compress, the bone screw 115 translates downwards until itbears on the bottom of the slot 188 in the inner insert 186. Then theload is transmitted from the inner insert 186 through the shear pins 108and 109 to the outer insert 187 and the nail stem 110 as describedpreviously.

In a further embodiment, damped micromotion may be achieved by one or aplurality of energy storage elements arranged in any series or parallelcombination, including but not limited to Belleville washers, coned-discwashers, wave washers, slotted washers, finger washers, split washers,curved washers, volute springs, or coil springs (standard, variablepitch, barrel, hourglass, or conical).

In one embodiment, (FIGS. 23 and 24), in a nail 200 micromotion anddynamising functionality may be achieved through a two-part assemblyduring the surgical procedure. The nail 200 is prepared for insertioninto the patient according to the standard operative technique. Aninsertion handle is attached to the nail 200 via a standard insertionbolt (not shown for simplicity), which has been modified to allow ananchor 206 to thread or press onto the end of the insertion bolt. Theanchor 206 has a central lumen to accommodate a guide wire and a taperedend to aid passage of the guide wire from the larger diameter of thenail lumen through the anchor 206. The surgeon checks the alignment of ahole 207 in the anchor 206 relative to a slot 220 in the nail stem 210and turns the insertion bolt to make any necessary rotationaladjustments before inserting the nail 200 into the intramedullary canal.After the nail has been implanted in the patient, the distal bonescrew(s) are inserted and a bone screw 115 is inserted through theproximal bone fragment, the slot 220 in the nail 200, and the hole 207in the anchor 206. The insertion bolt and insertion handle are thenremoved. Another component of the dynamisation unit comprises apre-assembled module containing a modified end cap 216, a shear pin 221,and a slider 209. This module is threaded simultaneously into both theinternal threads 211 in the proximal end of the nail stem 210 and theinternal threads 208 in the anchor 206. The dynamisation unit maycontain a single shear pin 221 for dynamisation or a plurality of shearpins. When the patient bears weight, the load is transmitted from thebone screw 115 through the anchor 206 to the shear pin 221 and theslider 209. As the top of the slot 215 in the slider 209 bears down onthe shear pin 221 and the load is transmitted through the modified endcap 216 to the nail stem 210 and the distal bone fragment. Theembodiment may also contain a Belleville washer or a plurality ofBelleville washers 201 to provide damped micromotion in the swing phase.

In any embodiment previously described, the correct rotational alignmentof the parts (including but not limited to the pre-assembleddynamisation unit relative to the nail) may be achieved by one or morehard-stops and may or may not be accompanied by one or more audibleclicks to signify to the surgeon that the device is properly inserted.

In another embodiment (FIGS. 25 and 26) in a nail 230, the micromotionand dynamisation functionalities of the invention are achieved via abeam-ratchet mechanism. Initially, a beam 233 or energy storage elementrests on the bearing surface of the uppermost ratchet as shown. The beam233 passes through a slot in a slider 232 to admit micromotion. If thepatient applies sufficient weight, the beam 233 snaps downwards to thenext ratchet increment, bringing the bone fragments closer together. Theload at which the beam 233 will progress to the next increment isdetermined by the beam 233 cross section (constant or non-constant),initial shape (flat or curved), and material properties. The beam mayalso comprise a plurality of beams such as a leaf spring. A ratchet 234profile, including the angle, depth, width, spacing, and orientation ofbearing surfaces and sliding surfaces shown in FIGS. 25 and 26 is givenfor illustrative purposes only to represent a series of grooves thatdifferentiate unique ratchet increments for bone separation distancecontrol. In this embodiment, the rotational orientation of theratchet-beam-slider sub-assembly is controlled relative to the insertsleeve 237 by a pin 236. After the nail is implanted in the patient andlocked distally, the insert sleeve 237 is threaded into the proximal endof the nail stem 238. The proximal bone screw 115 is then inserted andan end cap 231 is screwed into the nail stem 238 and/or the insertsleeve 237.

In a further embodiment (FIGS. 27 and 28), in a nail 240 thebeam-ratchet mechanism illustrated in FIGS. 25 and 26 is inverted andpositioned above the bone screw 115 to accommodate a different operativetechnique. In this instance, the complete dynamising unit ispre-assembled and comprises the following components. A beam 249 springelement passes through a slot in a slider 247 and is initiallypositioned in an uppermost groove 245 of a ratchet 243. Theslider-beam-ratchet unit is then affixed by a pin 245 or otherwisemounted inside a modified end cap 241. Two identical curved grips 250are mounted on the circular bearing lip 248 of the slider 247 and thegrips are pinned or otherwise fastened together (not shown forsimplicity). Thus, the grips 250 have rotational freedom relative to theend cap 241. During the operation, the surgeon implants the nail asnormal and inserts bone screws in the distal holes and proximal slot.The dynamising unit is threaded into the proximal end of the nail stem251 and curved grips 250 snap onto the bone screw 115 in a proximal slot252.

In a further embodiment (FIG. 29), the beam ratchet mechanismillustrated in FIGS. 25 to 28 is assembled inside the nail through atwo-part assembly similar to the procedure described for FIGS. 23 and24. In short, an anchor 261 is initially attached to an insertion boltuntil the proximal bone screw 115 is inserted. Then the insertion handleand bolt are removed, leaving behind the anchor 261. The dynamisationunit is then simultaneously threaded into the proximal end of the nailand the anchor 261. In this way, connectivity is achieved between theproximal and distal bone fragments.

In a further embodiment (FIGS. 30 and 31), in a nail 270 the ratchetmechanism is integral with a slider 273. The ratchet grooves arecontained in an upper sleeve 271 and are shown here with a squarecross-section for illustrative purposes only to represent a series ofgrooves that differentiate unique ratchet increments for bone separationdistance control. The upper sleeve 271 attaches to a nail stem 279 viascrew threads, pins, or other rigid fixation not shown in thisillustration for simplicity. The top of the slider 273 has a tapered lip272 that initially engages in the uppermost ratchet groove. The top ofthe slider 273 is also formed with a keyhole 280 or other cutout toallow the slider 273 to flex and snap from one ratchet position to thenext. In this embodiment, a plurality of Belleville washers 274 isincluded between the lower sleeve and the slider. The bone screw 115passes through a hole 277 in the lower sleeve 275 and a slot 276 in theslider 273 to admit micromotion. As the patient applies weight, theBelleville washers 274 deform and the bone screw 115 bears down on thebottom of the slot 276 in the slider 273. If the patient appliessufficient weight, the slider 273 progresses downwards to the nextratchet position, carrying with it the lower sleeve 275, the Bellevillewashers 274, and the bone screw 115 to close the gap between theproximal and distal bone fragments. In this embodiment, thefunctionality of the Belleville washers 274 is preserved at all ratchetpositions.

In one embodiment (FIGS. 32 and 33), in a nail 300 a ratchet mechanismcomprises a spring pin, split pin, or roll pin engaged in a series ofthrough-holes. A pre-assembled dynamisation unit comprises an outerinsert 306, an inner insert 302, and a spring pin 305. The spring pin305 initially passes through an uppermost through-hole 308 in the outerinsert 306 and a slot 311 in the inner insert 302 to admit micromotion.The dynamisation unit attaches to a nail stem 309 via screw threads,pins, or other rigid fixation not shown in this illustration forsimplicity. The bone screw 115 passes through slots 310 and 307 in thenail stem 309 and outer insert 306 and the hole 303 in the inner insert302. When the patient applies weight, the load is transmitted from thebone screw 115 to the inner insert 302 which bears down on the springpin 305 and transmits the load to the outer insert 306 and the nail stem309. If the patient applies sufficient weight, the spring pin 305 willsnap down to the next ratchet position, carrying with it the innerinsert 302 and the bone screw 115 to close the gap between the proximaland distal bone fragments. The spacing and profile of the through-holes308 comprising the ratchet are given by way of example only to representa series of unique ratchet increments for bone separation distancecontrol.

In one embodiment (FIGS. 34 and 35), in a nail 320 a bone separationcontrol mechanism is comprised of a direction-reversing spiral ratchetwith the following components. The bone screw 115 passes through a hole338 in a sleeve 336, which is attached to an insert 334 by a pin 339that initially rests in the rightmost recess of a spiral ratchet groove337 in the sleeve 336. A lip 333 of the insert 334 rests in a bearing322 of an end cap 321, so as to admit axial fixation and rotationalfreedom relative to the nail stem 327. When the patient applies weight,the load is transmitted from the bone screw 115 to the sleeve 336, whichbears down on the pin 339 and transmits the load via the insert 334, endcap 321, and nail stem 327 to the distal bone fragment. Dynamisation isachieved with the patient in a recumbent position, so the proximal bonefragment is considered fixed. The surgeon applies a distraction force tothe foot, which causes the nail stem 327, the end cap 321, the insert334, and the pin 339 to travel downwards relative to the fixed bonescrew 115 and the sleeve 336 with the spiral ratchet groove 337. The pin339 rotates and translates downward to the trough of the spiral ratchetgroove 337 and the when the surgeon releases the foot, the pin 339translates and rotates to the peak of the next ratchet increment, whichin turn brings the proximal and distal bone fragments closer together.The embodiment may or may not have a rotational spring element to ensurethat the sleeve 336 only rotates in one direction relative to the insert334. The shape and position of the spiral ratchet groove in the sleeveare shown here by way of example only. In a further embodiment, thespiral ratchet could be formed in the insert 334, with the tracking pinattached to the sleeve 336.

In one embodiment (FIGS. 36 and 37), in a nail 340 a bone separationcontrol mechanism is comprised of a pair of spring-resisted hingedplates. The bone screw 115 passes through a hole in a slider 345. A pairof plates 344 is pinned to the slider 345 and a rotational spring orother energy storage element embedded in or near the pinned jointprevents the plates 344 from freely rotating upward. In their initialposition, the pins are engaged in an uppermost groove 343 in an end cap342. The end cap 342 is threaded into the proximal end of a nail stem348. When the patient applies weight, the load is transmitted from thebone screw 115 to the slider 345 and the plates 344, which translatedownward in the groove 343 to generate micromotion. The plates 344 beardown on the bottom surface of the groove 343 and the load is transmittedto the end cap 342, the the nail stem 348, and the distal bone fragment.If the load applied exceeds the threshold of the resistive element atthe pinned joint, the plates 344 will rotate upward allowing the slider345 to progress downwards to the next groove 343 in the modified end cap342, thereby bringing the bone fragments closer together. The ratchetprofile, including the angle, depth, width, spacing, and orientation ofbearing surfaces and sliding surfaces shown in FIGS. 36 and 37 is givenfor illustrative purposes only to represent a series of grooves thatdifferentiate unique ratchet increments for bone separation distancecontrol. In a further embodiment, the hinged plates could be carried onthe end cap 342 and the ratchet grooves 343 formed on the slider 345.

In one embodiment (FIGS. 38 and 39), the device may include thecapability for post-hoc adjustment of the distance between the proximaland distal bone fragments. In a previous embodiment (FIGS. 15 to 17),the inner insert 106 transmitted loads from the bone screw 115 throughshear pins 108 and 109 to the outer insert 107, which was attached tothe nail stem 110 via the proximal screw threads 111 and 112. In thecurrent embodiment, in the nail 360 an inner insert assembly 361 may becomprised of three sub-components to facilitate rotational freedom of anouter insert 365 relative to the bone screw 115. A slotted sleeve 362has a circular bearing lip, which supports a similar circular bearinglip on a bone screw adapter 363. A threaded locking ring 364 holds theslotted sleeve 362 and the bone screw adapter 363 together and allowsthem to rotate relative to each other. The assembled inner insert 361,comprising the slotted sleeve 362, bone screw adapter 363, and lockingring 364, is then attached to the outer insert 365 via shear pins 108and 109. The device is threaded into proximal screw threads in the nailstem, which is then implanted into the patient. After the surgeoninserts the bone screw 115 through the nail stem 366 and the bone screwadapter 363, the axial position of the bone screw 115 may be adjusted byrotating the outer insert in the proximal screw threads. Thus, the outerinsert 365, the shear pins 108 and 109, and the slotted sleeve 362rotate together and translate axially in the proximal screw threads 367while the bone screw adapter 362 and bone screw 115 only translateaxially and do not rotate. In this way, the bone separation distance maybe adjusted after the bone screws have been inserted while preservingthe micromotion and dynamisation functionality of the device. When thebone separation distance has been set to the surgeon's satisfaction, theend cap is threaded into the proximal threads in the nail to fix theposition of the device.

The embodiments described are presented by way of example only. Each ofthe elements described in any of the embodiments may or may not bearranged in combination with elements from another embodiment to achievethe same effect or combination of effects. For example, any of themechanisms presented for bone separation control (including but notlimited to shear pins, beam ratchets, spring pin ratchets, spiralratchets, hinged ratchets) may be combined with any of the operativeassembly techniques (including but not limited to insertion of thepre-assembled module in the nail prior to implantation in the patient ortwo-part assembly of the module during implantation of the nail) andwith or without damped micromotion by one or a plurality of Bellevillewashers or other energy-absorbing elements arranged in any series orparallel combination. The illustrative figures provided show examples ofthe mechanical subcomponents arranged in a few selected configurationsand are not an exhaustive representation of all of the possiblepermutations of the subcomponents described herein. A few illustrativeexamples of these alternative permutations are now presented.

In one embodiment (FIGS. 40 and 41), a nail 400 comprises a single-stagepin-in-slot mechanism for controlling interfragmentary micromotion. Thedevice 400 has an end cap 402, pins 401, an outer insert 403, an innerinsert 404, and a stem 405. The pins 401 are large enough to withstandpatient weight bearing without shearing, so no dynamisation occurs. Thepin in slot allows controlled bone fragment separation as the shear pinpair 401 travels in the slot in the inner insert 404. Theinterfragmentary micromotion distance is chosen to accelerate callusformation and fracture healing by mechanical stimulation while providingcontrolled bone separation during both stance and swing phases viamechanical hard-stops. Damping of the motion between bone fragments isprovided by the inherent viscoelastic properties of the tissue in thebone gap.

In one embodiment (FIGS. 42 and 43), in a nail 411 damping actionprovided by the tissue in the bone gap may be augmented by the additionof a plurality of Belleville washers 410 or other energy-storageelements, which are compressed during stance phase and rebound to theirinitial shape during swing phase. The Belleville washers 410 rest on anouter insert 412 and are compressed by an inner insert 413, whichtransmits loads from the proximal bone screw 115.

In an alternative embodiment (FIGS. 44 and 45), the nail 420 maycomprise Belleville washers 421 seated on top of the outer insert 422and compressed by a disc 423 that is welded or otherwise affixed to aninner insert 424 which transmits loads from the bone screw 115.

In another embodiment (FIGS. 46 and 47), a nail 430 may comprise thepin-in-slot micromotion mechanism much as described in a previousembodiment (FIGS. 40 and 41) and is augmented to include the capabilityfor post-hoc adjustment of the bone gap as described in a previousembodiment (FIGS. 38 and 39). In this embodiment there are outer insert,inner insert, and bone screw adaptor 431, 433, and 432 in a stem 434. Inthis embodiment, the nail 430 an inner insert assembly 436 may becomprised of three sub-components to facilitate rotational freedom of anouter insert 431 relative to the bone screw 115. The inner insert 433has a circular bearing lip, which supports a similar circular bearinglip on a bone screw adapter 432. A threaded locking ring 437 holds theslotted sleeve 433 and the bone screw adapter 432 together and allowsthem to rotate relative to each other. The assembled inner insert 431,comprising the slotted sleeve 433, bone screw adapter 432, and lockingring 437, is then attached to the outer insert 431 via shear pins 435.The device is threaded into proximal screw threads in the nail stem 434,which is then implanted into the patient. After the surgeon inserts thebone screw 115 through the nail stem 434 and the bone screw adapter 432,the axial position of the bone screw 115 may be adjusted by rotating theouter insert in the proximal screw threads. Thus, the outer insert 431,the shear pins 435, and the slotted sleeve 433 rotate together andtranslate axially in the proximal screw threads 438 while the bone screwadapter 432 and bone screw 115 only translate axially and do not rotate.In this way, the bone separation distance may be adjusted after the bonescrews have been inserted while preserving the micromotion functionalityof the device. When the bone separation distance has been set to thesurgeon's satisfaction, the end cap is threaded into the proximalthreads in the nail to fix the position of the device.

In another embodiment (FIGS. 48 and 49), a nail 450 comprises a dampedmicromotion mechanism such as described in a previous embodiment (FIGS.44 and 45) and is augmented to include the capability for post-hocadjustment of the bone gap as described in a previous embodiment (FIGS.38 and 39). In descending order as illustrated the nail 450 comprises anend cap 451 with through holes 457, Belleville washers 452, a middleinsert 453, a bottom insert 454, in a stem 456. In the currentembodiment, the nail 456 an inner insert assembly 450 may be comprisedof three sub-components to facilitate rotational freedom of an outerinsert 453 relative to the bone screw 115. The inner insert 451 has acircular bearing lip, which supports a similar circular bearing lip on abone screw adapter 454. A threaded locking ring 458 holds the slottedsleeve 453 and the bone screw adapter 454 together and allows them torotate relative to each other. The assembled inner insert 450,comprising the slotted sleeve 451, bone screw adapter 454, and lockingring 4**, is then attached to the outer insert 453 via shear pins 459.The device is threaded into proximal screw threads in the nail stem 456,which is then implanted into the patient. After the surgeon inserts thebone screw 115 through the nail stem 456 and the bone screw adapter 454,the axial position of the bone screw 115 may be adjusted by rotating theouter insert in the proximal screw threads. Thus, the outer insert 453,the shear pins 459, and the slotted sleeve 451 rotate together andtranslate axially in the proximal screw threads 456(a) while the bonescrew adapter 454 and bone screw 115 only translate axially and do notrotate. In this way, the bone separation distance may be adjusted afterthe bone screws have been inserted while preserving the dampedmicromotion functionality of the device. When the bone separationdistance has been set to the surgeon's satisfaction, the end cap isthreaded into the proximal threads in the nail to fix the position ofthe device.

In another embodiment (FIGS. 50 and 51), a nail 460 comprises amulti-stage pin-in-slot mechanism for controlling interfragmentarymicromotion and providing non-surgical autodynamisation according to theshear pin concept described in a previous embodiment (for example. FIGS.16 and 17). The nail 460 also includes the capability for post-hocadjustment of the bone gap. The nail 460 comprises an end cap 461, upper462, middle 464, and lower 465 inserts, a washer 466, and a stem 467. Inthe current embodiment, the nail stem 467 an inner insert assembly 460may be comprised of three sub-components to facilitate rotationalfreedom of an outer insert 462 relative to the bone screw 115. The innerinsert 464 has a circular bearing lip, which supports a similar circularbearing lip on a bone screw adapter 465. A threaded locking ring 466holds the slotted sleeve 464 and the bone screw adapter 465 together andallows them to rotate relative to each other. The assembled inner insert460, comprising the slotted sleeve 464, bone screw adapter 465, andlocking ring 466, is then attached to the outer insert 462 via shearpins 463. The device is threaded into proximal screw threads in the nailstem 467, which is then implanted into the patient. After the surgeoninserts the bone screw 115 through the nail stem 467 and the bone screwadapter 465, the axial position of the bone screw 115 may be adjusted byrotating the outer insert in the proximal screw threads. Thus, the outerinsert 462, the shear pins 463, and the slotted sleeve 464 rotatetogether and translate axially in the proximal screw threads 468 whilethe bone screw adapter 465 and bone screw 115 only translate axially anddo not rotate. In this way, the bone separation distance may be adjustedafter the bone screws have been inserted while preserving themicromotion and dynamisation functionality of the device. When the boneseparation distance has been set to the surgeon's satisfaction, the endcap 461 is threaded into the proximal threads in the nail to fix theposition of the device.

In another embodiment (FIGS. 52 and 53), a nail 500 comprises a tube 507that allows interfragmentary micromotion by controlling the alignment ofbone screws in oversized holes. The tube contains multiple through-holes501, 502 and 503 of different diameters that are aligned with fixationholes 505, and 506 and a dynamic slot 504 in a nail stem 510. The middlethrough-hole 502 has a diameter equal to the width of the dynamic slot504 in the nail 510. The upper and lower through holes 501 and 503 havea diameter slightly smaller than the diameter of the holes 506 and 505in the nail stem 510. To lock the micromotion-enabled nail, the surgeoninserts a large bone screw 115 through the stem slot 504 and middlethrough-hole 502 to provide rotational fixation for the tube and toguarantee purely axial micromotion. The surgeon also inserts two smallerbone screws through either or both the upper and lower through-holes506/501 and 505/503. The difference in the diameter between the holes inthe nail and the holes in the tube determines the micromotion distance.In this example the bone screw 115 diameter is 4 mm, passing through anover-size hole 505 of 5 mm, giving a micromotion distance of 1 mm.

In an alternative embodiment, either the upper or lower through-holes inthe tube can be made equal in diameter to the corresponding hole in thenail, so the surgeon has the option to insert a large bone screw andengage rigid fixation without micromotion. In the embodiments discussedabove, damping of the motion between bone fragments is provided by theinherent viscoelastic properties of the tissue in and around the bonegap.

In a further embodiment (FIGS. 54 and 55), in a nail 520 damping actionprovided by the tissue may be augmented by the addition of a pluralityof Belleville washers 521 or other energy-storage elements, which arecompressed during stance phase and rebound to their initial shape duringswing phase. The Belleville washers 521 rest on a plug 522 and arecompressed by a tube 523, which transmits loads from the proximal bonescrews 115 extending through a stem 524.

In an alternative embodiment the Belleville washers 521 may be seated ona pre machined shoulder in the nail and are compressed by the tube,which transmits loads from the bone screws. In any of the embodimentsdiscussed above, the holes for the bone screws may be aligned in asingle plane or in a series of oblique planes rotated about thelongitudinal axis of the nail.

Referring to FIG. 56 a nail 540 has a stem 541 and an insert 542. Theinsert 542 has through holes 543 and 544, and the stem has acorresponding through-hole 546 and a slot 547. The insert 542 also hasslots 552, into which keys 551 are fitted. The stem 541 has an axialgroove 550 in which the keys 551 slide to ensure that the insert 542 hasonly axial translation relative to the stem 541 and no rotationalfreedom. The through-hole 546 in the insert 542 ensures that the bonescrew inserted through the stem is perfectly normal to the axis, asillustrated in FIG. 58. In another embodiment the groove may be in theinsert 542 and the slots in the stem 541.

It will be appreciated from the embodiments of FIGS. 52 to 56 thatmicromotion can be allowed by using a bone screw that has a smallerdiameter than the hole in the nail that it passes through. The amount ofmicromotion in this configuration is determined by the difference indiameter between the bone screw and the hole in the nail. The insertensures that the screw does not enter the hole off-axis, which wouldresult in no micromotion. The insert guides the bone screw into correctalignment within the hole as needed to ensure micromotion. Axial loadspass directly from the bone screw to the nail, not through the insert.In this embodiment, the insert is merely a surgical guide to aid bonescrew alignment and it does not take axial forces.

It will be appreciated that additional rotational stability could beadvantageous to bone healing and could be achieved in several ways:

-   -   Reducing the width of the slot in the nail stem would prevent        bone screw rotation and admit axial micromotion only with no        rotational freedom.    -   Including a keyway that aligns the insert inside the nail stem        would ensure the nail and insert have only axial translation        relative to one another and no rotational freedom.    -   Forming the lumen of the nail stem with an elliptical        cross-section and the corresponding insert with the same        cross-section would ensure that nail and insert only have axial        translation relative to one another and no rotational freedom.

In the first option presented above for providing additional rotationalstability, the insert prevents skewed insertion of the bone screw asshown in FIG. 58, but the insert itself is not load bearing for eitheraxial or torsional loads. In the second and third options above, theinsert is load bearing under torsional loads, but not load bearing underaxial loads.

In one embodiment, one or more of the holes in the nail stem may beelongated to form a short slot with width equal to the diameter of thebone screw and length equal to the diameter of the bone screw plus therequired interfragmentary micromotion distance. To lock themicromotion-enabled nail, the surgeon inserts a bone screw through theshort slot to provide torsional stability with axial micromotion. Asecond bone screw may also be added in a longer dynamic slot.

It will be appreciated that the embodiments described herein can bemanufactured using known techniques. For example, the components of themotion assembly shown in FIGS. 15 to 18 comprise two computernumerically controlled (CNC) machined tubes 106 and 107 with theaddition of slots 120 and 121 by wire electrical discharge machining(EDM) and an externally machined thread on tube 107. Pins 108 arestandard turned shafts. The device is assembled by sliding tube 106 into107 and pressing pins 108 into slots 106, 107 respectively. In FIGS. 19to 22, variations of the embodiment are presented to facilitate springdamping by means of Belleville washers, which are standard machinecomponents not requiring unknown manufacturing techniques and having noaffect on the manufacturing of the components previously described. Inanother embodiment shown in FIGS. 52 and 53, a CNC machined tube 507 hasa number of holes 501, 502, and 503, each produced according to standardtechniques.

The invention is not limited to the embodiment hereinbefore described,with reference to the accompanying drawings, which may be varied inconstruction and detail. For example the micromotion assembly mayinclude a spring of a different type such as a coil spring or a leafspring. The micromotion assembly may include non-spring dampers andrestoring force systems such as elastic, macromolecular, visco-elastic,magnetic, electric, electromagnetic, pneumatic, hydraulic,oleo-pneumatic, electro- and magneto-hydrodynamic pumping systems. Also,resilience may be provided by the inherent properties of part of thenail. For example, a polymer sleeve for the bone-fastening screws may beresilient, thus providing the required spring resilience and/or damping.

Also, alternative mechanisms could be used to limit separation of thebone fragments during the swing phase. Any locking mechanism which isstrong enough to withstand the necessary forces and which engages at aset position could be used. It need not necessarily be a ratchetmechanism as it may be required in some embodiments to allowbi-directional movement for re-adjustment. For example, an arrangementakin to the ball-and-spring arrangement employed in crutches may beemployed.

Also, it is envisaged that there may be more than one micromotionassembly in the nail. Also, its orientation could be adjustable.

The improvements compared to a standard interlocked intramedullary nailthat are facilitated by the embodiments presented in detail herein canalso be achieved through different embodiments, which are given here byway of example, with reference to the accompanying drawings.

The invention claimed is:
 1. An intramedullary nail comprising: a nailstem; a proximal bone screw and a distal bone screw; an insert beingshorter than the nail stem, completely within the nail stem and havingan aperture to receive said proximal bone screw for engagement with afirst bone fragment; the nail stem having a distal aperture to receivesaid distal bone screw for engagement with a second bone fragment; thenail stem having a proximal aperture arranged for alignment with theaperture to receive said proximal bone screw, the insert being adaptedto guide insertion of the proximal bone screw through the nail stemproximal aperture without taking any axial load from the proximal bonescrew, and the insert being constrained to move axially only within thenail stem without relative rotation of the insert and the nail stem; andwherein, the nail is adapted to allow limited and repeated relativeaxial movement of less than 1.5 mm of the insert relative to the nailstem, said limited and repeated relative movement being determined by adifference in cross-sectional size between the proximal bone screw andthe nail stem proximal aperture.
 2. The intramedullary nail as claimedin claim 1, wherein the insert is keyed in the stem.
 3. Theintramedullary nail as claimed in claim 1, wherein the nail is adaptedto provide spring bias and/or damping between the insert and the nailstem.
 4. The intramedullary nail as claimed in claim 1, wherein the nailis adapted to provide spring bias and/or damping between the insert andthe nail stem, wherein the nail comprises a removable spring.
 5. Theintramedullary nail as claimed in claim 1, wherein the nail is adaptedto provide spring bias and/or damping between the insert and the nailstem, wherein the nail comprises a removable spring, and wherein thespring comprises a plurality of spring elements.
 6. The intramedullarynail as claimed in claim 1, wherein the nail is adapted to providespring bias and/or damping between the insert and the nail stem whereinthe spring or spring elements comprise one or more Belleville washers.7. The intramedullary nail as claimed in claim 1, wherein the nail isadapted to allow said relative movement without bias or damping.
 8. Theintramedullary nail as claimed in claim 1, wherein the nail is adaptedto allow adjustment of an interfragmentary gap within which saidrelative movement occurs by varying minimum and/or maximum separation ofthe proximal and distal bone fragments.
 9. The intramedullary nail asclaimed in claim 1, wherein the nail comprises a surgeon adjustmentinterface which translates rotational movement caused by a surgeon intoaxial movement within the nail stem to close the gap between theproximal and distal bone fragments.
 10. The intramedullary nail asclaimed in claim 1, wherein the nail comprises a surgeon adjustmentinterface which translates rotational movement caused by a surgeon intoaxial movement within the nail stem to close the gap between theproximal and distal bone fragments, wherein the interface comprises anexposed screw which pushes an internal sliding and non-rotatingcomponent within the nail stem.
 11. The intramedullary nail as claimedin claim 1, wherein there is a plurality of proximal bone screws. 12.The intramedullary nail as claimed in claim 1, wherein there is aplurality of distal bone screws.